Invasive snails and slugs are among the most important pests of agriculture, and horticulture including high-value crops. They incur direct losses in crop yield and quality (fecal and mucus contamination) and increase management costs; and are vectors of plant and human pathogens, e.g., Alternaria brassicicola, pathogen of brassica dark leaf spot and Angiostrongylus cantonensis that causes eosinophilous meningo-encephalitis. Management of slugs and snails relies almost exclusively on molluscicides, with metaldehyde use currently being the standard practice. Although generally effective and efficient, it is very toxic to pets and under high relative humidity, activity is reduced and snail recovery possible. Iron phosphate and sodium ferric EDTA are often used in high humidity or where there are concerns about pet safety. However, iron phosphate may also be toxic to non-targets like earthworms. Methiocarb is a restricted use material and is toxic to mammals including humans, birds, bees and aquatic taxa. Other nonbiological control options, e.g., copper barriers, are impractical for large areas.
Following the rediscovery of Phasmarhabiditis hermaphrodita in the UK in 1996, Phasmarhabditis was reported in Chile (France and Gerding, 2000), Iran (Karimi et al., 2003), Egypt (Genena et al., 2009), New Zealand (Wilson et al., 2012), China (Huang et al., 2015; NCBI sequence data) and the USA (Tandingan De Ley et al., 2014). There are currently six nominal species of the genus with P. hermaphrodita having the widest host range and association with 15 gastropod families (see Grewal et al., 2002; Rae et al., 2007; Ross et al., 2011). P. hermaphrodita with bacteria Moraxella osloensis has been commercially sold as Nemaslug®, a biopesticide, in the UK and 14 European countries for over 20 years.
P. hermaphrodita was recently reported in California from three invasive slug taxa Arion hortensis agg., Deroceras reticulatum and Lehmannia valentiana (Tandingan De Ley et al., 2014). During the same gastropod survey, two other Phasmarhabditis species were recovered: Phasmarhabditis papillosa from a single Deroceras reticulatum specimen and a new species, Phasmarhabditis californica isolated from three slug taxa (Arion hortensis agg., Deroceras reticulatum, and Lehmannia valentiana).
Prior to the introduction of molecular tools, species diagnostics in Phasmarhabditis was based exclusively on: i) female tail shape and length; ii) appearance of the bursa and spicule length; iii) number and position of bursal papillae; and iv) frequency of males and host association. As with other nematode groups, few diagnostic morphological features define each species. Females of different species are difficult to separate morphologically and males are very rare except for P. papillosa. Additionally, papillar arrangement on the male bursa is known to be diverse within highly studied rhabditid model organisms Caenorhabditis elegans and C. briggsae, with male individuals of the latter exhibiting patterns typical of the former (Baird, 2001). This underscores the importance of finding stable taxonomic characters to define a species and of using a combined taxonomic approach for an efficient and accurate species diagnostics.
The discovery of Phasmarhabditis for the first time in the US and North America paved the way for studies on its biological control potential against invasive slugs and snails including one that has not been introduced to California but has become economically important in Florida and Hawaii.